Horizontal vs. Vertical Filter Control

I'm currently moderating a RITE study at my company for a top 10 mutual fund firm. Part of our goal for the site is to make it easy for people to quickly find a set of relevant funds. For the first iteration we employed a filter tool horizontally above the results set like so:



We were concerned that people wouldn't know that the filter options were cumulative, or that they'd be thrown off when the results changed on every click of a filter. However, users told us something a bit more obvious. That is, the results set was partially below the fold so it was cumbersome to view a new results set.

Bring in the beauty of the RITE study where we could try a whole new approach. We put the filter options on the left site of the page like so:



Users had no problem using this version. Because the client was there all along, we didn't have to convince anyone that this approach was best. The users proved it for us.